Demodulation system



se t,-z1,1943. M. CAWEIN 2,329,877

DEMODULATION SYSTEM Filed Sept. 29, 1941 INVENTOR MADSON CAWEW TTORNEYPatented Sept. 21, 1943 UNITED sTArEs PATENT oFFicE 2,329,817; j j

7 Madison Oawein, Marion, worth Television and R poration of Delaware V7 Application September 29, 1941, Serial No. 412,732

'4 Claims. (01. 250521) t I v fore, to provide a new and improvedcontrollable This invention relates to a demodulation system andparticularly to a demodulation system having a controllable output.

Demodulation systems conventionally used in modern receivers ofmodulated carrier Waves generally comprise a diode rectifier tubeconnected in series relation with a load circuit comprising a fixedresistor shunted by a fixed condenser. Since theintensity of receivedcarrier Waves can vary greatly, the intensity of the demodulated signaldeveloped across the load circuit of the diode rectifier also variesaccordingly. However, as the reproducing device, to which thedemodulated signal is finally applied, sometimes after amplificationthereof, operates best within a limited range of input signalintensities, it is necessary to provide a control between thedemodulation system and the repro-' ducing device in order to apply tothe reproducing device an input signal extending over a desired rangeonly.

In television receivers, it has been customary heretofore to provide apotentiometer between the demodulation system and the cathode raypicture reproducing tube, by means of which the intensity of the controlsignal applied to the cathode ray tube can be adjusted for the purposeof controlling the contrast of the reproducedtelevision image. Such apotentiometer is usually connected in the load circuit of the dioderectifier tube or in the load circuit of an amplifier tube coupled tothe diode rectifier and has its sliding contact connected to thecontrolgrid of an amplifier tube or of the cathode ray picture reproducingtube. Since there always exists a certain amount of distributed capacityacross the resistance winding of such a potentiometer, a varying amountthereof according to the potentiometer adjustment is connected in shuntrelation to the control grid-cathode space of the tube, to whose controlgrid the sliding contact is connected. For different settings of thesliding contact of the potentiometer therefore, the input admittance ofthe tube following the potentiometer is varied, which gives rise tochanges in the frequency response characteristics of the channeltransmitting the demodulated signal. Such changes are highlyundesirable, particularly in a television signal channel which isusually corrected for substantially uniform frequency response over avery wide band of frequencies, because any changes in circuit constantsin the channel tend to impair the frequency responsecharacteristics.

It is an object of the present.invention,"therepedance comprising aInd., asjsignor to Farn sadio Corporation, acordemodulation system; freefrom the disadvansofthe arrangements hitherto used. I'

In accordance with. the present invention, there is provided a-controllable" demodulation system having a pair ofinput terminalsadapted to have a modulated carrier signal applied there'- to, havinghalf-waves of opposite polaritiesi- A first circuit is providedcomprising a first uni lateral impedance means and a load impedanceconnected between the input terminals. The first unilateral impedance menal half-waves of one polarity only,

ans is responsive to sigacross the load impedance. There-is alsoprovided a second circuit "co'niprisinga second unilateral impedancemeans, a variable impedance means, and the load-circuit,"connectedbetween the inputterminals, The second unilateral impedancemeans isresponsive to signal half waves of the opposite polarityonly, thereby'todevelop across the load impedance a demodulated signal of'oppositepolarity and'of an intensity dep ending upon the value ofthe variableimpedance.

Finally, there is provided apair of output terminals coupled to'the'load impedance for deriving a demodulated signal of controllableintensity therefrom;

For a better understanding of the invention; together with other andfurther objects thereof," following description, the accompanying Iimpedance means, such as the diode rectifier tube 3, and a load imresistor '4; connected in shunt relation with a condenser li." Asecondjcir cuit is also connected between the'input terminals I and 2comprising a second unilateral impedance means, such as the diode 6,connected inserie's relation with a variable a variable resistor 1shunted and also including terminals 9 and I0 pedance comprising therebyto develop a demodulated'signal of one polarity demodulation system emthere is shown a con and 2 is a first circuit" unpedance comprising by acondensen'fl, 1 the I load v impedance comprising-resistor4 andcondenser 5. A'pair of output" are coupled to the load im'-' a resistor4 and condenser" the applied carrier signal only. The voltage defveloped across the condenser 5 is of such polarity that. the cathode ofthe diode 3 176116831205 become.

increasingly positive. The ratev at which this voltageincreases isdetermined'by theinipeda nce of the charging circuit of the condenser 5;which comprises the signal source (not shown) connected to the inputterminalsv l' and 2' and the diode 3. Obviously, the voltage developedacross the condenser 5 at the end of each positive half wavciofrthecarrier signal denendsoni the im.- p danceot its charging: circuit.Durin r thcme ative hal -wave he conden er: 5 disch r zacross.

the resistor 4;, as is well ls-ncwnitzo those skille im he art. Inthis-mamien a. demodulated; sign l:

corncsmndingto, the modulation envelope orthop sit-liv carrierhalt-waves; is developed. across the load: impedance comprisingcondenser; E; and

resistor 41';

Thedi d 6., i not. responsive to positive can rier 'halfi-w a ssince t:isccnn ct d; withl polartr opn siteto thatiof the; diode 3;. and itconuc saonlyduring the ne tive hafrWEYCS-Of the carri r" signal; ndcloses, a. secondcircuit. betweenv thc'inputz rmina s l nd 2;whichialsoi d s the: condenser Again, a, current is caused; to flowthrough-1 the; condenser 5, whose directiom. however; is: opposite.tothe. condenser current, flowin -throu h thedicde 3 durinszm itivehalf- The voltage developed; acro s: the condenser 5. bythe; current,flowing through, the; diode: l5:- isof pposite; p la i y t the v l a edeveloped; by the; positive carrier signal half-waves and tends tomake!the: anodeof. the diode; 65 negative, condenser. currentand the voltagedeveloped, across. condenser 5 during; each: negative, hal-f wave againdepends on the impedance; of the,

ondenser char ing, c rcuit wh chnow inc udes the: source of carrier,signal (not shown-ls. the;

diode 6;and thevar-iable impcdan egelement; comprising, the variableresistor T; shuntedby; the

condenser 8. If the resistor-l iscompletelx-shert.

circuited, the voltage developed across the con.- denser 5,, due tothenegative halfewaveawill; be equal and;oppositetorthevoltage developedthere--v across by the positive half-wavese If, however, the resistor Ihas a finite-value, the'chargingcurrent, during the negative halt-wavesand, therefore. the. voltage developed acrossthecondenser 5, is.decreased. Again, obviously, the condenser charges, due to the negativehali wavesrleak off;

through shunt resistor t duringrtheipositive halt waves. It; is evident,therefore, that a. demodulated: signal is developed across; the loadimped ance, comprising resistor 4. and condenser 5. The polarity of thissignal is opposite to that of the demodulated signal producedbythe'positive carrier halt-waves, whereby the resulting; demodu, latedsignal is ofcontrollable intensity ranging between zero andayaluedetermlnediby the;val-ue of theresistor 1-. Thecondenser-8 has1beenproevided tor by -passing the. carrier iltcduency but is nctgessentiai toratisfa tory operation; of; the; rranssmcnt. l

The resulting demodulated signal of controllable intensity is developedacross the output terminals 9 and H! which can be coupled to the controlgrid and cathode, respectively, of an amplifier tube or a reproducingdevice, such as a cathode ray picture reproducing tube. Since the outputof the demodulation system is controllable, i1; is obvious that no otherco ,trolsare required between the; demodulation system and a reproducingdevice, so that the circuit constants of the modulation signal frequencychannel remain unchanged when the magnitude of the demodulated nal ischae d.

Whilethere has been described what is at Dresent' considered thepreferred embodiment of the inventionit willibe obvious to those skilledin the art that various changes and modifications may be. madethereinwithout departing from' the invcnticmand it is, therefore, aimed in theappended' claims to cover all such changes and modifications as. fall;within the true spirit and scope, of theinvention,

What is claimed is;

1.. controllable demodula ion sys em adapted tolhave amodulated arriersignalapplied thereto hev n rhalf Wavesof different polarities compriseing: rectifying: means and-- a fixed time constant circuit; connectedin,- series-therewith for develop? n a. d modulat d; si nal of? e parity a oss 1 said fixedtime constant circuit, a second rectify-- ingmoans anda variable impedance means connected: in; series with eachother across said first rectifying means for normally developing ademodulated signal of opposite polarity across said time; constantcircuit of an; intensity depending upon the value of said variableimpedance means and controlling; the-intensity of thewholedemodlatedsisna 2. A- controllable demodulation system com prising a.pair ofinput terminals adapted to-have a modulated carrier signalapplied thereto; having; half-waves. of opposite polarity, a pair ofoutput terminals, a load impedance connected between said outputterminals and a. demodula- ,tion circuit connected between theinputterminals and the output terminals comprising a.

means.- responsive to signal half-waves of one polarity only fordeveloping across said load impedance a, demodulated signal of onepolarity, and means connected to .said input terminals and said; loadimpedance responsive to signal half- Waves, of the opposite polarityonly for developing; a, demodulated signal across said load impedanceofopposite polarity and of an intensity dependent uponthe valueof saidvariable im-.

pedance means to thereby control the intensity of, thewhole; demodulatedsign al-.

3, A. controllable demodulation; system comprising a pair of inputterminals adapted to. havea modulated carrier signal appliedtheretohaving. half-waves oioppositev polarity, a pair of output.terminals, a load impedance connectedbetween said output terminals and ademodulas tion, circuit connected between one of; said input.

erminals and one of said output terminals com prising, a firstunilateral, impedance means re sponsive to signali half-waives of onepolarity only for" developing across; said load; impedance a demod latedsignal of one polarity, anda series including a, second: unilateral impeance means: responsive to. signal hair-waves oi the pposite polar ty'only. and a variable impedance means. connected across said firstunilateral in, pedance means for developing a demodulated 2 ignal:across.- said; load impedance of pposite polarity and of anin'tensity'dependent upon the value of said variable impedance meansthereby to control the amplitude lated signal.

4. A. controllable demodulation system comprising a pair of inputterminals adapted to have a modulated carrier signal applied theretohaving half-waves of opposite polarity, an output terminal connected toone of said input terminals, another output terminal, a load impedanceconnected between said output terminals and a demodulation circuitconnected between the other of said input terminals and the other ofsaid output terminals comprising a first uniof the Whole demodulateralimpedance means responsive to signal impedance means thereby to controlthe amplitude of the Whole demodulated signal.

- MADISON CAWEIN.

and a series circuit including a

